Thermal stir welding apparatus

ABSTRACT

A welding method and apparatus are provided for forming a weld joint between first and second elements of a workpiece. The method includes heating the first and second elements to form an interface of material in a plasticized or melted state interface between the elements. The interface material is then allowed to cool to a plasticized state if previously in a melted state. The interface material, while in the plasticized state, is then mixed, for example, using a grinding/extruding process, to remove any dendritic-type weld microstructures introduced into the interface material during the heating process.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of application Ser. No. 09/994,506 filedon Nov. 27, 2001 now abandoned.

ORIGIN OF THE INVENTION

This invention was made by employees of the United States Government andmay be manufactured and used by or for the Government for governmentalpurposes without the payment of any royalties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a welding method and apparatus, and inparticular, to a welding method and apparatus which separatelyplasticizes or melts the surfaces to be joined followed by a subsequentweld matrix mixing process.

2. Background of the Invention

Welding processes can be classified into one of two categories, fusionwelding and solid state welding. Fusion welding involves meltingmaterial to be welded and includes such processes as MIG, TIG and VPPAwelding. Solid state welding joins materials without a melting step andinclude the processes of friction stir and inertia welding. Fusion weldprocesses typically result in a dendritic type weld microstructureexhibiting inferior mechanical and structural properties. Such inferiormaterial properties are generally seen in metals subsequent to melting.Conversely, solid state weld processes result in a non-dendritic grainstructure exhibiting properties superior to those produced with fusionwelding processes.

Both fusion welding and solid state welding have respective limitations.As indicted above, fusion welding compromises the microstructure of thematerial and thus lessens the physical properties and characteristics ofthe material. Solid state welding such as inertia welding is limited torounded structures such as pipe or rod structures.

A recent advancement was made when friction stir welding becameavailable for the solid state welding of materials. Reference is made,for example, to U.S. Pat. Nos. 6,168,067 B1 to Waldron et al. and6,053,391 to Heideman et al. With the use of friction stir welding, asolid state weld could, for the first time, be provided in applicationsrequiring longitudinal welds, ranging from several inches to anunlimited length. As described in more detail in the aforementionedpatents, the friction stir weld process uses a rotating shoulder/pinconfiguration. The shoulder produces frictional heat to bring thematerial into a plasticized state and forges the welded material withextremely large forces. To accomplish the necessary large forces for theforging effect, a very robust backing anvil is required for support. Thewelding pin spins inside the workpiece at the same rate as the shoulder.The dependent motion of the welding pin and shoulder restricts the speedof the welding process.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention, a welding method and apparatusare provided for forming a weld joint. The method separates the weldingprocess into separate and discrete steps, one providing heating of firstand second abutting elements to be welded to form an interfacetherebetween of plasticized or melted material and a further matrixtransformation step for processing the interface material after coolingto a plasticized phase. The heating process may use any conventionalfusion welding process such as laser or plasma torch which initiallymelts the interface material. Subsequently, a separategrinding/extrusion element recrystallizes possible, undesirabledendritic matrix structures while the interface material is in theplasticized temperature state. Advantageously, separate heating sourcescan be used to plasticize or melt the respective first and secondelements forming the weld joint independently. The independent meltingfeature provides for the joining of dissimilar metals such as copper andaluminum or stainless steel and titanium.

According to one aspect of the present invention, a welding method isprovided for joining a workpiece comprising first and second elements inabutting relation along facing surfaces. The method includes heating thefirst and second element to plasticize or melt the elements at least atthe facing surfaces so as to form an interface therebetween of materialin a melted state. If the interface material is heated to a meltedstate, then the interface material is allowed to cool from the meltedstate to a plasticized state. Next, the interface material is mixedwhile in the plasticized state.

According to another aspect of the present invention, an apparatus isprovided for forming a weld joint in a workpiece between first andsecond elements in abutting relation along facing surfaces. Theapparatus includes a heating device for plasticizing or melting thefirst and second elements at least at the facing surfaces so as to forminterface therebetween of material in a plasticized or melted staterespectively. A mixing tool mixes the interface material when in aplasticized state.

Preferably, the apparatus further comprises forming means for exertingforce on the elements to control forming thereof. Advantageously, thetemperature of the forming means is controlled to provide heating orcooling of the elements.

In one preferred embodiment, the forming means comprises at least oneforging plate. Advantageously, the apparatus further comprises controlmeans for sensing the force exerted by the forging plate and forcontrolling feeding of the first and second elements based thereon.Preferably, the control means controls one of feed rate and travel speedto control feeding of the elements.

In another preferred embodiment, the forming means comprises a pluralityof rollers. In this embodiment, the apparatus preferably furthercomprises control means for controlling the force exerted by the rollersand for controlling feeding of the first and second elements basedthereon. Preferably, the control means controls one of feed rate andtravel speed to control feeding of said elements.

Preferably, the apparatus further comprises control means for sensingenergy input to the heating elements and for controlling one of feedrate or travel speed of the elements based thereon.

In a preferred implementation, the apparatus further comprises apre-weld tack welding means located upstream of the mixing tool for tackwelding the two elements together prior to mixing by the mixing tool.

Advantageously, the mixing tool is retractable so as to enable completewithdrawal thereof from the first and second elements.

Preferably, the apparatus further comprises containment forging platesfor containing the first and second elements during mixing by the mixingtool.

According to yet another aspect of the present invention, a joinedworkpiece has a first element comprising a first metal material having afirst plasticized temperature and a second element comprising a secondmetal material having a second plasticized temperature different fromthe first plasticized temperature. A longitudinal weld joint is formedbetween the first metal element and the second metal element. The weldjoint has a recrystallized fine grain matrix.

A key feature of the present invention relates to the separation of aheating process to form a plasticized or melted interface materialbetween two elements to be joined and a mixing process for mixing theinterface material together while in a plasticized state. Thisseparation enables each of the respective materials being joined to beheated and plasticized/melted independently. One advantage of thisfeature is that the invention enables dissimilar metals to be weldedtogether which previously were not able to be joined, i.e., metals suchas copper/aluminum, stainless steel/copper, and stainlesssteel/titanium. For example, a copper/aluminum weld can be achieved byproviding independent temperature control as each alloy is brought up toits respective plasticized/melted state. A further advantage ofseparating the heating process from the weld matrix transformationprocess is that comparatively high workpiece travel rates during weldingcan be obtained. In this regard, the present invention is not inherentlylimited with respective to travel rates as is the case with a frictionstir welding process.

An additional important feature of the present invention is that theinvention provides matrix transformation of the interface material fromdendritic to fine grained material. More specifically, the presentinvention provides for transforming grain structure from thedendritic-type weld microstructure formed as a result of the heatingprocess to a recrystallized fine, non-dendritic grain structure. As aconsequence, the resulting final grain structure is typically that of asolid state weld material and thus exhibits excellent mechanicalmaterial properties.

A further advantage of the present invention is that the invention canprovide long longitudinal welds of varying material thickness.

Yet another advantage of the present invention is that the invention canbe used to form longitudinal welds exhibiting a solid state weldmaterial property without a backing anvil such as is required in stirwelding.

Further features and advantages of the present invention will be setforth in, or apparent from, the detailed description of preferredembodiments thereof which follows.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a longitudinal cross sectional view of a welding apparatusaccording to a preferred embodiment of the present invention;

FIG. 2 is an end elevational view of the welding apparatus of FIG. 1;and

FIG. 3 is a longitudinal cross sectional view, partially broken away, ofa further embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1 and FIG. 2, there is shown a welding device,generally denoted 10, which is adapted to join together first and secondelements 14 and 16 of a workpiece 12. The welding device 10 includesheating elements 18 and 20.

Heating element 18 provides sufficient heat to plasticize or melt thematerial of element 14 and may comprise such conventional heatingdevices as lasers or plasma torches or other suitable devices known inthe art. Similarly, heating element 20 provides sufficient heat toplasticize or melt the material of element 16. As a result, together,heating elements 18 and 20 transform a portion of the solid material ofthe workpiece 12, viz., respective abutting portions of elements 14, 16to form an interface 34 (see FIG. 2) in a plasticized or melted phase,between the elements 14, 16.

The heating elements 18 and 20 can be controlled individually forproviding heating at a desired separate temperature. For example, whereelements 14 and 16 comprise dissimilar materials having differentmelting points, heating element 18, preferably provides heating at theplasticizing or melting temperature of element 14 whereas heatingelement 20 preferably provides heating at the plasticizing or meltingtemperature of the material of element 16.

The material of element 14 and element 16 may be formed of the same ordifferent metal material. For example, element 14 may be copper orstainless steel, and element 16 may be aluminum, copper or titanium, sothat when joined together, elements 14 and 16 form a workpiece 12 formedof copper/aluminum, or stainless steel/copper or stainlesssteel/titanium or another combination.

In an alternative embodiment, additional heating elements (not shown) ofthe type of heating elements 18, 20 may be disposed adjacent to theheating elements 18, 20 and/or below workpiece 12, to assist inplasticizing/melting the abutting portions of the elements 14, 16. Anoptional tack weld heater 22, disposed upstream, relative to the heaters18 and 20, provides sufficient heat to form an initial tack weld betweenthe elements 14, 16.

As shown in FIG. 2, a pair of clamping elements 36, 38 apply arespective force on the workpiece 12 towards each other. The forceapplied maintains elements 14 and 16 in proper alignment with eachother.

A mixing tool such as toothed grinding/extruding member 40 is positionedin the path of the interface 34 between elements 14 and 16 and rotatesto mix the material of interface 34 when in a plasticized phase. Thegrinding/extruding member 40 allows plasticized metal of the interface34 to flow and extrude through the individual teeth, which are indicatedat 40 a, of member 40. The teeth 40 a are angled, thus forcing theplasticized material to flow downwards or upwards depending upon theangle of the teeth 40 a. As a result, the grinding/extruding member 40will recrystallize the dendritic matrix structure which may have beenformed in the material of interface 34 as a result of the meltingprocess. Grinding/extruding member 40 is retractable as indicated byarrow A and can be completely withdrawn from the workpiece formed byelements 14 and 16.

A plurality of force actuators 42 are located downstream of grindingmember 40 and used to apply a force on a pair of forging plates 44, 46which are located on opposite sides of workpiece 12. Force actuators 42include rollers 42 a which engage and bear against plates 44, 46. In analternative embodiment, plates 44, 46 can be eliminated and rollers 42 aused to bear directly on the workpiece, i.e., either rollers, or plates,can be used separately to exert the required force to further form theworkpiece. Further, the rollers or forging plates can be heated orcooled (e.g., by water cooling) to control the temperature of theworkpiece material.

As indicated in the drawings, the heating elements described above andthe grinding member 40 extend through forging plates 44, 46 so as topermit them to perform their respective functions. The force exertedupon the forging plates 44, 46 by the force actuators 42 is constant fora workpiece having constant thickness, while a variable force is exertedon the forging plates 44, 46 by the actuators 42 to accommodateworkpieces of a tapered thickness.

A pair of motion control devices indicated separately at 48 control theamount of movement of the forging plates 44, 46. The motion controldevices 48 each may comprise a LVDT, a laser device or other suitablemotion control device known in the art.

During the operation of the welding device 10, the workpiece 12 isinserted into an entrance 50 of a housing 52 which houses the variouselements and units described above. As set forth hereinbefore, theheating elements 18, 20 provide the desired heating of elements 14 and16, to form the plasticized or melted phase interface 34.

During the heating process, undesirable dendritic-type Weldmicrostructures may be introduced into the matrix of the interfacematerial 34. Advantageously, the heating process takes place in an inertenvironment. For example, nitrogen gas can be pumped into housing 52 toprovide an inert environment, thereby reducing or eliminating theoxidation of the material of workpiece 12 during the heating process.

The workpiece 12 proceeds through the welding device 10 in a directiondenoted by arrow 54. The material of interface 34 is in a plasticized ormelted state while in the area generally indicated by reference numeral56. As the workpiece 12 proceeds along direction 54, if the interfacewas heated to a melted state, the melted interface is transformed into aplasticized state. The transition point where the melted interfacebecomes plasticized is denoted by a dashed line 58. The workpiece 12transitions from the melted state to the plasticized state due to theabsence of applied heat. Alternatively, if the material of interface 34is merely heated to a plasticized state, the interface material remainsin a plasticized state as the workpiece proceeds past dashed line 58.

The workpiece 12 then proceeds to the location the grinding/extrudingmember 40. The interface 34, now in the plasticized state, is processedby the grinding/extruding teeth 40 a of member 40 and the plasticizedmaterial of interface 34 flows and is extruded through the teeth 40 a.As indicated above, this processing of the interface materialdramatically recrystallizes the grain structure, thus producing a finegrained weld matrix when fully cooled.

As shown in the drawings, member 40 includes a central mixing portioncomprising the grinding/extruding teeth 40 a extending along the lengththereof and support portions directly connected to the central portion.Considering, for example, the upper support portion, this portionextends through an opening in forging plate 44 which terminates at amajor surface at plate 44, as illustrated.

The workpiece 12 next travels past the grinding/extruding member 40 to alocation where the hot interface material, which is still in aplasticized state, is forged under pressure by the forging plates 44,46.

In an advantageous embodiment, a controller or control system 60 isemployed which controls the feed rate or travel speed of the workpieceformed by elements 14 and 16 by controlling the force exerted by eitherthe forging plates 44, 46 and/or rollers 42 a (whether used separatelyor in combination, as indicated schematically by the dashed lines inFIG. 1). Although a separate controller 60 is shown, the control systemcould directly control force actuators 42.

Alternatively, or in addition, a control system or controller 62 is alsoprovided which controls the material feed rate or travel speed bymonitoring or sensing the energy input to the heating elements 18 and20, or as illustrated, the energy input to a further heating element 64.The overall control system could also include a sensor 66 for sensingfeed rate or travel speed and supplying a corresponding input signal tocontroller 62.

Referring to FIG. 3, in accordance with a further important feature ofthe invention, separate containment forging plates 68 and 70 areprovided closely adjacent to or, in one embodiment, in surroundingrelation to, the mixing tool 40 so as to contain the heated material ofelements 14, 16 during rotation of the grinding teeth 40 a of mixingtool 40.

It should be apparent to those of ordinary skill that the present deviceand process offers important advantages over previous welding methods.These advantages include the ability to weld together dissimilar alloyswhich previously could not be joined due to differences in theirrespective melted and plasticized phase temperatures. Further, theseparation of the plasticizing/melting process and interface matrixtransformation process results in significantly enhanced welding speeds.

Although the invention has been described above in relation to preferredembodiments thereof, it will be understood by those skilled in the artthat variations and modifications can be effected in these preferredembodiments without departing from the scope and spirit of theinvention.

1. An apparatus for forming a weld joint in a workpiece, said apparatuscomprising: a support assembly supporting first and second longitudinalelements, as the first and second elements are moved longitudinally tothe support assembly, in abutting relation along laterally opposedfacing edge surfaces which opposed edge surfaces form an intendedlongitudinal weld joint of the workpiece; a heating device for supplyingheat to provide plasticizing of the first and second elements at leastat a zone of the intended longitudinal weld joint as the first andsecond longitudinal elements are longitudinally moved past the heatingdevice, so as to form a continuous longitudinal interface between theopposed facing edge surfaces of interface material in a plasticizedstate; an elongate mixing tool mounted downstream of the heating deviceso as to extend between the first and second longitudinal elements, saidmixing tool extending through the plasticized interface material from afirst side of the first and second longitudinal elements to a second,opposite side thereof, and said mixing tool mixing the plasticizedinterface material thereat as the first and second longitudinal elementsare moved past said mixing tool whereby a longitudinal weld joint isformed downstream from said mixing tool; a first control means forcontrolling the heating device so that sufficient heat is provided atthe zone to the interface material to ensure that the interface materialarriving at said mixing tool is sufficiently plasticized for mixing bythe mixing tool to subsequently form the longitudinal weld joint; and asecond control means for controlling the mixing tool so that mixing ofthe plasticized interface material by said mixing tool subsequentlyforms the longitudinal weld joint; and wherein said support assemblyincludes first and second, separately controlled, containment forgingplates respectively positioned on said first and second sides of thefirst and second longitudinal elements, said containment forging plates,as the first and second longitudinal elements are longitudinally movedbetween and relative to said forging plates, a) containing the movingfirst and second longitudinal elements during mixing by said mixingtool, and b) effecting forging of the moving first and secondlongitudinal elements downstream of the mixing tool, said containmentforging plates each including one major surface thereof in engagementwith a respective side of said first and second longitudinal elements sothat the first and second longitudinal elements are contained onopposite sides thereof between said containment forging plates, andwherein said mixing tool is separate from said first and secondcontainment forging plates, includes a central mixing portion locatedbetween the containment forging plates, and extending transverselybetween the containment forging plates from one containment forgingplate to the other, includes at least one support portion connecteddirectly to the mixing portion and extending through an opening in oneof the containment forging plates, the opening terminating at thecorresponding major surface of the one containment forging plate inengagement with a surface of one of the first and second longitudinalelements, and includes teeth along a length of said central mixingportion which teeth effect the mixing of the plasticized interfacematerial to form the weld joint.
 2. The apparatus of claim 1 whereinsaid heating device comprises first and second separate heat sources. 3.The apparatus of claim 1, wherein said mixing tool comprises a grindingand extruding device.
 4. The apparatus of claim 1, further comprising ahousing for maintaining an inert environment for the first and secondelements when plasticized by said heating device.
 5. An apparatus ofclaim 1, further comprising control means for sensing energy input tosaid heating device and for controlling one of feed rate and travelspeed of said longitudinal elements based thereon.
 6. An apparatus ofclaim 1, further comprising a pre-weld tack welding means locatedupstream of said mixing tool for tack welding the two longitudinalelements together prior to plasticizing by said heating device andmixing by said mixing tool.
 7. An apparatus of claim 1, wherein saidmixing tool is retractable so as to enable complete withdrawal thereoffrom the first and second longitudinal elements.
 8. The apparatus ofclaim 1, wherein said heating device applies heat to the first elementat a first element welding temperature and applies heat to the secondelement at a second element welding temperature different than the firstwelding temperature.
 9. The apparatus of claim 8 wherein said heatingdevice comprises a first heat source for applying heat to said firstelement and a second, separate heat source for applying heat to saidsecond element.
 10. The apparatus of claim 9 wherein said first andsecond heat sources each comprise a laser.
 11. An apparatus of claim 1,further comprising separately controlled forming means for exertingforce on said longitudinal elements to control forming thereof andseparate control means for controlling said forming means.
 12. Anapparatus of claim 11, wherein the temperature of said forming means iscontrolled to provide heating or cooling of said longitudinal elements.13. An apparatus of claim 11, further comprising control means forsensing the force exerted by said at least one forging plate and forcontrolling feeding of said first and second longitudinal elements basedthereon.
 14. An apparatus of claim 13, wherein said control meanscontrols one of feed rate and travel speed to control feeding of saidlongitudinal elements.
 15. An apparatus of claim 11, wherein saidforming means comprises a plurality of rollers.
 16. An apparatus ofclaim 15, wherein said separate control means comprises control meansfor controlling the force exerted by said rollers and for controllingfeeding of said first and second longitudinal elements based thereon.17. An apparatus of claim 16, wherein said control means controls one offeed rate and travel speed to control feeding of said longitudinalelements.